1. Field of the Invention
The present invention relates to an improved ion beam sputtering apparatus suitable for forming an electrically conductive film on a semiconductor layer as a constituent in semiconductor devices including a photoelectric conversion device such as a photovoltaic element (or a solar cell), a line or area photosensor, and a thin film transistor (TFT), specifically said thin film semiconductor having one or more semiconductor junctions of pin, nip, pn, or np formed of a non-single crystalline silicon material such as an amorphous silicon (a-Si) material, a microcrystalline silicon (xcexcc-Si) material or a polycrystalline silicon (poly-Si) material.
The present invention also relates to a method for forming a transparent and electrically conductive film on an improved electrically conductive substrate having a given semiconductor layer and a process producing a semiconductor device including a photoelectric conversion device such as a photovoltaic element (or a solar cell), a line or area photosensor and a thin film transistor (TFT), having an improved electrically conductive film.
2. Related Background Art
It is known that a thin film semiconductor device may be produced in a manner wherein a thin film semiconductor layer is formed on a substrate having an electrically conductive film formed thereon, an electrically conductive film is formed on said thin film semiconductor layer, followed by fabricating into an element. For instance, in the case of producing a photoelectric conversion device such as a solar cell (or a photovoltaic element), a thin film semiconductor layer having one or more semiconductor junctions of pin, nip pn, or np is formed on an electrically conductive surface of a substrate, and a transparent and electrically conductive film is formed on said thin film semiconductor layer.
It is known that the formation of such electrically conductive film on the thin film semiconductor layer may be conducted by way of vacuum evaporation or DC magnetron sputtering.
In the case of a photovoltaic element comprising a semiconductor layer having a desired semiconductor junction formed on an electrically conductive substrate through a light reflection layer and a transparent and electrically conductive film formed on said semiconductor layer, the transparent and electrically conductive film usually is comprised of In2O3, SnO2, ZnO, or ITO (In2O3+SnO2). The transparent and electrically conductive film comprising any of these materials may be formed by way of vacuum evaporation, sputtering, CVD or spraying process. Particularly, a transparent and electrically conductive film comprising ITO (hereinafter referred to as ITO film) excels in light-transmittance and electrical conductivity and has an excellent etching property. Because of this, the ITO film is widely used. For the formation of the ITO film, DC magnetron sputtering is frequently used in view of significance in the property of the film formed and also in the productivity.
Now, in order for the photovoltaic element to effectively exhibit a desirable electromotive force, the transparent and electrically conductive film used therein is required to have a desirably low electric resistance and a desirably high light-transmittance. However, in the case where an ITO film is formed on the semiconductor layer formed on the electrically conductive substrate through the light reflection layer at a thickness of 500 to 1000 xc3x85 which is suitable for the transparent and electrically conductive film on said semiconductor layer under conditions of a relatively low substrate temperature of 230xc2x0 C. or less and a deposition rate of 10 xc3x85/sec. or above which is desirable in terms of productivity, a sheet resistance of only about 50 xcexa9/xe2x96xa1 can be attained for the resulting transparent and electrically conductive film.
Japanese Unexamined Patent Publication No. 331413/1995 (hereinafter referred to as Document 1) discloses a method of forming a transparent and electrically conductive film on a specific compound semiconductor layer of CuInSe2 or the like having a homo junction formed on an electrically conductive substrate by sputtering a sintered ZnO target and a sintered Al2O3 in an atmosphere composed of O2 gas or H2 gas by way of ion beam sputtering using Ar ion beams.
The present inventor conducted experimental studies of the method described in Document 1. As a results, this method was found to still have such subjects as will be described in the following, which are necessary to be improved in order to form a desirable electrically conductive film on a given thin film semiconductor layer in accordance with the ion beam sputtering method described in Document 1.
That is, the results of experimental studies by the present inventor revealed such findings as will be described in the following. In the production of a photovoltaic element, when an ITO film as a transparent and electrically conductive film is formed on a thin film semiconductor layer formed of a non-single crystalline silicon material and having a pin junction formed on an electrically conductive substrate in accordance with the ion beam sputtering method described in Document 1, direct current (which is considered due to negative charge flown from the surface of the thin film semiconductor layer to the electrically conductive substrate) is liable to flow from the electrically conductive substrate toward the surface of the thin film semiconductor layer, where the direct current is converged at an electrically weak portion (which is rather lower in electric resistance than its periphery) to cause breakdown (short-circuit) in the thin film semiconductor layer situated in the vicinity of the electrically weak portion. This situation results in making the resulting photovoltaic element to have an insufficient performance which is poor in stability upon repeated use over a long period of time.
The present inventor conducted experimental studies of the reason why such short-circuit occurred in the thin film semiconductor layer. As a result, there were obtained such findings as will be described in the following. That is, particularly in the production of a photovoltaic element using a long substrate web made of stainless steel as an electrically conductive substrate, after a thin film semiconductor layer formed of a non-single crystalline silicon material and having a pin junction is formed on said long substrate web using the conventional roll-to-roll system, when an ITO film as a transparent and electrically conductive film is formed on the thin film semiconductor layer in accordance with the ion beam sputtering method described in Document 1 and using the conventional roll-to-roll system, problems are liable to entail in that since the supporting and transporting rollers of the roll-to-roll system are constituted by a metal which is electrically conductive, the substrate web (made of stainless steel) is eventually electrically grounded when it is contacted with the supporting and transporting rollers, where electric field is generated between the surface potential of the thin film semiconductor layer and the earth potential of the substrate web upon the ion beam sputtering to form the ITO film, the electric field is impressed into the thin film semiconductor layer wherein electric current is convergently flown through a portion having a lower electric resistance present in the thin film semiconductor layer from the substrate web (the electrically conductive substrate) toward the surface of the thin film semiconductor layer whereby causing such short-circuit as above described between the substrate web and the ITO film as the transparent and electrically conductive film formed on the thin film semiconductor layer. The resulting photovoltaic element becomes to have an insufficient performance which is poor in stability upon repeated use over a long period of time.
Herein, the ion beam sputtering is stronger than the DC magnetron sputtering and the like in terms of the intensity of plasma energy. Hence, the above problem of causing short-circuit is more liable to occur in the case of the ion beam sputtering in comparison with the case of the DC magnetron sputtering. In addition, in the case of the ion beam sputtering, the thin film semiconductor layer is more liable to suffer from plasma damage during the formation of the transparent and electrically conductive film thereon.
An object of the present invention is to provide an ion beam sputtering apparatus improved so as to effectively prevent the occurrence of the foregoing problem of causing such short-circuit in the prior art and a process which enables to efficiently produce a semiconductor device.
Another object of the present invention is to provide an improved ion beam sputtering apparatus in which a long electrically conductive substrate web having a semiconductor layer formed thereon can be effectively subjected to treatment by way of ion beam sputtering while continuously moving said substrate web and while preventing the semiconductor layer on the substrate web from suffering from plasma damage and a process which enables to effectively produce a high quality semiconductor device, said process including a step of forming a desirable electrically conductive film on the semiconductor layer formed on the substrate web by way of ion beam sputtering based on the ion beam sputtering apparatus.
A further object of the present invention is to provide an improved ion beam sputtering apparatus which enables to effectively form a transparent and electrically conductive film comprising grains having a substantially uniform size densely distributed therein and which has a desirably low electric resistance and a high light-transmittance, excels in acid proof and alkali proof, stands with high temperature, and is always stable even upon repeated use over a long period of time, and a process for producing a semiconductor device in which said transparent and electrically conductive film is used.
A further object of the present invention is to provide an ion beam sputtering apparatus comprising a first means for generating an ion beam and directing said ion beam in a prescribed direction, a second means for supporting a target at a position where said target is capable of exposing said ion beam irradiated in said prescribed direction and of being sputtered by said ion beam, a third means for supporting an electrically conductive substrate having a semiconductor layer on which a component sputtered from said target is to be deposited, and a fourth means for making said electrically conductive substrate to have a non-earth potential.
A further object of the present invention is to provide a method for forming a transparent and electrically conductive film on an electrically conductive substrate having a semiconductor layer thereon, said method comprising the steps of: arranging a target at a position in an ion beam sputtering apparatus where said target is irradiated with an ion beam, arranging said electrically conductive substrate at a position in said ion beam sputtering apparatus where a component sputtered from said target is deposited on said semiconductor layer of said electrically conductive substrate, and generating said ion beam and directing the ion beam generated in a prescribed direction to irradiate the ion beam to said target to sputter the target whereby depositing the component sputtered from the target on the semiconductor layer of the electrically conductive substrate whereby forming said transparent and electrically conductive film on the semiconductor layer, wherein at least during when the component sputtered from the target is being deposited on the semiconductor layer of the electrically conductive substrate, the electrically conductive substrate is controlled to a non-earth potential.
A further object of the present invention is to provide a process for producing a semiconductor device, comprising the steps of arranging a target at a position in an ion beam sputtering apparatus where said target is to be irradiated with an ion beam, arranging an electrically conductive substrate having a semiconductor layer for said semiconductor device at a position in said ion beam sputtering apparatus where a component sputtered from said target is to be deposited on said semiconductor layer on said electrically conductive substrate, and generating said ion beam and directing the ion beam generated in a prescribed direction to irradiate the ion beam to said target to sputter the target whereby depositing the component sputtered from the target on the semiconductor layer on the electrically conductive substrate, wherein at least during when the component sputtered from the target is being deposited on the semiconductor layer on the electrically conductive substrate, the electrically conductive substrate is controlled to a non-earth potential.
The semiconductor device herein includes a photoelectric conversion device such as a photovoltaic element or a solar cell, a line or area photosensor, and a thin film transistor (TFT).